Con Coughlin is the Telegraph's Defence Editor and a world-renowned expert on global security and terrorism issues. He is the author of several critically acclaimed books. His new book, Churchill's First War: Young Winston and the fight against the Taliban, is published by Macmillan in London and Thomas Dunne Books in New York. He appears regularly on radio and television in Britain and America.

Eric Joyce's resignation is another blow to the government's credibility over Afghanistan

When it comes to sorting out Afghanistan it seems that this government just can't get anything right. It has taken Downing Street three years to work out why British soldiers are risking their lives on the God-forsaken wastes of southern Afghanistan – to keep the streets of Britain safe from Islamist terror attacks – but no sooner has Gordon Brown issued a clear mission statement than a key New Labour ally tenders his resignation in protest.

As a former officer in the Black Watch, Eric Joyce, the former private secretary to Bob Ainsworth, the defence secretary, is well-placed to assess the progress we are making in Afghanistan. And while I take issue with his claim that our military presence in Helmand does not improve our national security – it most certainly does – he is correct to criticise the government for not having a proper exit strategy.

For the past three years, since British forces were first deployed to Helmand, the government has given the distinct impression that it is making up strategy as it goes along. This criticism applies to the entire Nato effort, of which Britain is a part, and is a weakness that American President Barack Obama is keen to rectify.

Rather than keeping Washington's commitment to Afghanistan open-ended, Mr Obama is determined to set clear objectives that must be attained within a clearly-defined time frame. And to show he means business, he has put the military command structure in place to make sure his goals are met.

Instead of arguing about whether to send an extra few troops, the British government should follow the American example and make sure our commanders have the resources they need to fulfil their mission objectives within a clearly-set time frame.